This application claims the priority of German Application No. 19749939.2, filed Nov. 11, 1997, the disclosure of which is expressly incorporated by reference herein.
The invention concerns a method of machining workpieces with rotationally symmetrical surfaces, for example unstable workpieces of a complicated shape, with rotationally symmetrical, even eccentric surfaces, and apparatus for such a machining procedure.
Machining is used in this specification broadly, embracing not just chip-cutting machining but also for example water jet cutting, laser cutting, laser hardening, heat treatment and so forth.
A typical representative of such a workpiece is a crankshaft used for reciprocating piston internal combustion engines, reciprocating piston compressors and so forth, or a camshaft for similar uses.
Machining of a crankshaft in the rough condition, that is to say cast or forged, consisting of steel or cast iron, is generally effected by known cutting machining procedures. Crankshafts are most frequently used in an internal combustion engine of a motor vehicle, and are generally produced in very large numbers. Therefore, in terms of selecting the machining method and the machine configuration, the method which has the expectation of the shortest possible machining time for each crankshaft is adopted.
In accordance with the known procedures, the central or main bearings of the crankshaft are machined by means of rotational broaching or turning-rotational broaching, for example as disclosed in German Patent DE 35 23 274 C2 or European Patent EP No 86 108 666, while the crank throw or big-end bearings and is possibly also the crank side cheek faces can be machined by means of external milling, in particular high-speed external milling, for example as disclosed in German Patent DE 196 26 627 A1, or by means of internal milling, also referred to as spinning cutting, or by means of rotary milling, for example as disclosed in German Patent DE 44 46 475.
As an alternative to external milling it, is also possible to use rotary milling. Under some circumstances the respective cutting machining operation takes place when the workpiece is in an already hardened condition. In that respect the terms just used are employed to denote the following:
Rotational Broaching:
Arranged at the periphery of a disk-shaped tool, spaced in the peripheral direction thereof, are rotational broaching cutting edges whose spacing increases relative to the center of the rotational disk-shaped tool. That disk-shaped tool rotates with its axis in parallel relationship beside the axis of the crankshaft and material is removed at the peripheral surface of the crankshaft by the rotational broaching cutting edges being pivoted along the periphery of the crankshaft which is rotating substantially faster (about 1000 rpm). If the rotational broaching cutting edges are all at the same spacing relative to the center of the tool, a feed must be implemented radially with respect to the crankshaft, in the X-direction, between the tool cutting edges. Those procedures can be distributed to a plurality of cutting edges or can be supplemented by sister tools.
Turning-rotational Broaching:
This involves the rotational broaching operation described above, wherein implemented prior thereto is a plunge-cut turning operation which is implemented by means of a cutting edge which is also arranged on the periphery of the disk-shaped tool. Plunge-cut turning is effected by a procedure whereby the disk-shaped tool does not rotate during engagement of the cutting edge, but is only moved radially forwardly towards the workpiece.
External Milling:
In this case also the cutting edges are disposed on the periphery of a disk-shaped tool which is drivable in rotation with an axis parallel to the axis of the workpiece. The cutting speed however results primarily from the rotary movement of the tool while the workpiece only rotates at about 10 rpm until an at least complete rotary movement of the tool has been completed about the rotationally symmetrical surface of the workpiece, which is to be machined.
Particularly when dealing with large oversizes, a number of passes of the tool around the workpiece surface are required, but even if a single pass seems adequate by virtue of the oversize involved, more than one complete rotary pass is often necessary because of the tangential inward and outward movement of the tool.
The disk-shaped tool is equipped with milling teeth over its entire periphery.
The spacing of the cutting edges in the peripheral direction relative to each other can possibly be less than in the case of rotational broaching or turning-rotational broaching, in regard. to which the intention is generally to conclude the machining operation with the one cutting edge before the next cutting edge comes into the condition of engagement into the workpiece.
Disk-shaped Tool:
This generally involves a circular disk. Theoretically however it is also possible to use non-circular disks, for example ellipses and so forth. Preferably however the disks only ever exhibit convexly outwardly curved peripheral contours and in that respect in particular do not have any hard or abrupt steps in the peripheral contour. If there are cavities in the peripheral contour, they are not equipped with cutting edges.
Rotary Milling:
In contrast to external milling, rotary milling is operated with a generally finger-shaped milling cutter whose axis of rotation is in orthogonal relationship with the axis of rotation of the workpiece to be machined. The peripheral surfaces are machined with the one or more end cutting edges of such a finger milling cutter, and the end faces of the workpiece are milled with the cutting edges arranged on the peripheral surface of the finger cutter.
High-speed Milling (Rotary Milling or External Milling):
This milling occurs at a cutting speed of, for example, in the case of steel: over 130 m/min, in particular over 180 m/min, in the case of cast iron: over 150 m/min, in particular over 200 m/min, and in the case of aluminum: over 300 m/min, in particular over 500 m/min. Such cutting speeds are promoted in particular by a positive cutting edge geometry and the appropriate cutting material choice.
This high cutting speed is advantageous because it minimises all the disadvantages of interrupted cutting, which are inherent in the milling system.
In the prior art, machining operations involving turning/rotational broaching/turning-rotational broaching on the one hand and machining by means of external milling or rotary milling, that is to say generally milling, on the other hand, were not used in combination as it was considered to be impractical by virtue of the completely different necessary ranges of rotary speed for the workpiece. While, in the case of turning/rotational broaching/turning-rotational broaching, the cutting speed was primarily attained on the basis of the speed of rotation of the workpiece which is about 1000 rpm for a private automobile crapkshaft, and the disk-shaped tool was pivoted in or rotated only at a speed of less than 30 rpm, the situation is approximately diametrally opposite in the case of external milling/rotary milling, in particular in the case of high-speed milling.
In a corresponding fashion the problems which occur in such machining procedures also arise in completely different areas:
In the case of turning/rotational broaching/turning-rotational broaching, it is not necessary to use the C-axis which involves monitoring the rotational position of the workpiece because of the high speed of rotation of the workpiece. Furthermore, co-ordinated tracking, of the tool in the X-direction is in any case not possible with that speed.
The main difficulties with the high speeds of rotation involved include the area of the clamping force, compensating for unbalance and so forth.
By contrast, with external milling/rotary milling, as inter alia the crank throw or big-end bearings are to be machined hereby, implementation of the C-axis is an absolute necessity. The problems lie in sufficiently rigidly clamping or supporting the workpiece and in holding the large heavy workpiece only on one side in a stable and accurate fashion. Particularly when high levels of accuracy are involved, a problem arises with true running and balanced condition of the tool and the tool shaft.
An object of the present invention is to provide a method for machining workpieces with rotationally symmetrical, or even eccentric surfaces such as camshafts or crankshafts, which provides for minimizing the setting and idle times, wherein, in particular, transposition of the workpiece on to another machine by changing the chucking mounting thereof is avoided, and wherein both large and small batch sizes are economically machined.
Another object of the invention is to provide apparatus for machining workpieces, such as crankshafts, which affords enhanced use versatility with a simplified and rational operating procedure.
The foregoing and other objects are attained by the method and apparatus according to the invention as set forth herein.
As will be apparent from the following description of preferred embodiments of the invention, by virtue of the fact that two procedures which are different in principle, as regards the effect of the relative speed between the tool and the workpiece, are used on the workpiece, the machining mode which is the most advantageous both technically and also economically can be selected for the respective operating location and thus considered in its totality over the workpiece. If the two procedures are applied on one and the same machine, considerable setting and idle times for transposition of the workpiece onto another machine, as well as the machining inaccuracies which are caused by re-chucking the workpiece on the other machine can be avoided.
Preferably the workpiece is also not completely released and then clamped again within the machine, but, if possible, the clamping or chucking action is only temporarily released on one side of the crankshaft.
It is thus possible to machine, for example, the central or main bearings of a crankshaft by rotational broaching or turning-rotational broaching, and obviously also by simple turning, while the crank throw bearings, under some circumstances in the same clamping condition, can be machined by means of external milling or rotary milling, and likewise the crankshaft side cheek faces.
Additionally, if there is the possibility of accommodating the crankshaft between points or centers and applying the torque necessary for the machining operation, then the end flange and the end journal of the crankshaft can be machined within the same machine, preferably by means of turning or rotational broaching or turning-rotational broaching, in order to implement clamping of the crankshaft by means of clamping jaws at the periphery thereof, at those surfaces which have then already been machined.
Chucks which are particularly suitable for this purpose have clamping jaws which engage the periphery of the workpiece which can be axially advanced and retracted relative to the point or the center which is guided at the center, namely a tailstock center. This enables a transition received between centers to be clamped at the periphery by means of clamping jaws, and even hybrid forms of those two options.
Preferably the tool units are also displaceable in the Y-direction and possibly also rotatable about the B-axis while there can also be additional boring and milling units on a tool support.
For that purpose a suitable machine preferably has clamping chucks which have both clamping points or centers and also clamping jaws. In this structure, the points are displaceable relative to the jaws in the axial direction, that is to say the Z-direction, in such a way that, upon clamping between points, the jaws, which are then retracted, do not impede machining at the periphery of the end journal and the end flange.
Such a machine has two machining units of which one permits the machining process of turning or rotational broaching or turning-rotational broaching or a combination thereof, while the other permits external milling or rotary milling, in particular at high speed. Those two machining units are arranged in particular on separate or separately actuatable supports which are displaceable at least in the X-direction, preferably also in the Y-direction, and possibly rotatable about the B-axis.
The machining units can each be. arranged on the same side of the workpiece or on opposite sides or at an angle relative to each other. In addition the tool units must also be displaceable in the Z-direction along the workpiece in order to be able to implement their machining procedure at different axial positions of the workpiece.
In addition to or instead of using one of the machining units, it is possible to provide a non-chip-cutting machining unit. This can be a water jet cutting device, a laser cutting device or a device for heat treatment, in particular a laser hardening device. Such a non-chip-cutting machining unit is also preferably displaceable in the Z-direction, possibly also in the X- and the Y-direction.
A movement in the X-direction is however not absolutely necessary depending on the respective machining method involved because, instead of a change in the radial spacing between the tool unit and the workpiece, for example in the case of a laser, it is only necessary to adjust the focusing. A similar procedure is also possible in the case of water jet cutting.
This combination of machining methods or machining units affords the possibility of producing even very small batch sizes and individual items of a given crankshaft at a still viable price. Therefore it is immaterial that the machining time in this case is a multiple of the machining time in the case of large-scale mass production of crankshafts.
Hardening in the machine, for example, by means of a laser beam or an inductor, for particularly the bearing surfaces, can also be used because it is possible to avoid releasing the workpiece, passing it through a separate hardening procedure and re-clamping the crankshaft in another machine in which the post-machining operation after hardening, in particular grinding, is completed. In addition, the hardening operation is effected only at locations where it is required, so that the heat-induced distortion, which occurs in the crankshaft, can be minimized or controlled in a deliberate and targeted fashion, insofar as given regions of the crankshaft are subjected to the heating and cooling effect in a given fashion in terms of magnitude, time sequence and time pattern.
A further advantage of hardening, for examples by means of a laser beam within the machine is that, even during the heating of regions of the crankshaft, which is required for the hardening procedure, by means of a laser beam, cutting machining is possible in those heated regions by means of one of the above-mentioned procedures. This drastically reduces the forces that occur in the cutting machining operation. In that respect the cutting machining procedure can also take place in the cooling period or can extend into, the cooling period so that, during simultaneous cooling, and cutting machining, the thermally induced distortion in the final product can be kept at a minimum level.
In order to be able to implement the extremely different speeds of rotation of the spindle stock or stocks for driving the workpiece, as are necessary with the two groups of the method, these drives may have for example, aside from a motor spindle or motor, an intermediate transmission which can be shifted or which can be brought into operation and which may have a step-up ratio or step-down ratio of the order of magnitude of at least 1:50, particularly 1:70, and preferably 1:100. Implementation of the C-axis on the spindle stock which only has to be involved in the slower mode of operation can also be coupled to the shiftable intermediate transmission. The transmission may be a worm gear/worm assembly or a planetary assembly.